Noncurling coated abrasive product and method of making the same



Patented Nov. 8, 1938 UNITED STATES NONCURLING COATED ABRASIVE PRODUCTAND METHOD OF MAKING THE SAME Nicholas Ewing Oglesby, Troy, N. Y.,assignor to Behr-Mannin g Corporation, Troy,

N. Y., a

corporation of Massachusetts No Drawing.

Application November 17, 1934, Se-

rial No. 753,546. Renewed October 12, 1937 14 Claims.

This invention relates to coated abrasives, and more particularly to anew method of producing coated abrasive articles and the productresulting therefrom. The primary object of the invention is theproduction of coated abrasive products, such as sheets, strips, discs,and the like, which are characterized by a reduction or elimination oithe usual tendency to curl or cup after the articles have beencompleted.

w Heretofore, in the manufacture of coated abrasives, such as sandpaper,it has been a common trouble to experience what is known as cupping ofthe coated abrasive sheets and belts. By cupping it is meant that thebelt or sheet, as the 15 case may be, does not lie flat afterfabrication, but curls or cups on an axis either parallel or at rightangles to the machine direction of the paper. This cupping or curlingmay occur: 1. With the sand coating on the convex surface,

2. With the sand coating on the concave surface.

Generally speaking, what is lmown as closed coats, or tight (closelyarranged) grain coatings, curl with the sand on the convex surface,whereas open coatings, or coatings where there is considerable spacebetween the grains, curl with the abrasive grains on the concavesurface. In either event, serious difilculty arises in using the productsince it is normally desired to sand a flat surface.

In the case of a belt with a sand concave curl, the tendency is to cutinto the work deeply on the edges of the belt and to out less deeplywith the center of the belt. It the sand is on the convex surface, thesituation is reversed, a deep cup being produced in the center of thebelt and a more shallow out being produced on the edges 01' the belt.Not only is this true, but breakage of belts often can be traced tocurling of the coated belt.

In cases 0! sheets that are used for hand sanding operations, it is alsodesirable to have a flat surface, the handling of a curled sheet beingvery inconvenient and resulting in decreased production as well as apoorer quality or work.

In the manufacture of coated abrasives, it is common practice to apply acoat of adhesive, generally animal glue, to a paper backing (for beltuse, normally cylinder paper) and then to apply a coat oi abrasivegrains, such, for instance, as

Garnet or Alundum, partially dry the first adhesive coat and then applya second or sizing coat of adhesive. After the second adhesive coat isapplied, the coated web is dried and taken down in the form of rolls forsubsequent slitting into belts or cutting into sheets.

The curling of the belts or the sheets, as the case may be, is caused byunequal expansion or contraction of the paper backing on the one handand the sand-glue coat, on the other hand, when exposed to variablehumidities. Both glue and 5 paper absorb and lose moisture according tothe changes in the humidity of the atmosphere. There is associated withthis gain or loss of moisture, as the case may be, an expansion orcontraction of the paper and of the glue. The 10 abrasive grains do notchange their dimensions with humidity. It the coated abrasive web isflat 'at some humidity such as 55% relative humidity and a temperatureof F., and the product is subsequently exposed to an atmosphere of 30%15 relative humidity at 70 F., both the paper and the glue lose moistureand contract. If the gluesand mixture contracts more than the paper, asand concave curl will result, whereas it the paper contracts more thanthe sand-glue coat, a sand 20 convex curl will result.

I have discovered that in order to produce a product which will be flatat all reasonable humidities, it is necessary that the paper and thesand-glue coat contract or expand substantially 25 the same amount, withloss or gain of moisture.

In other words, the paper and the sand-glue coat should havesubstantially the same expansioncontraction characteristic under similarchanges in the moisture content of the surrounding atso mosphere. I

In the case of closed coats, that is, coats with I closely spacedgrains, the paper heretofore used tends to contract more than thesand-glue coat with a drop in moisture content resulting from a drop inhumidity, with the result that a sand convex curl is produced. Likewise,in the case of very open coatings, that is coatings with only about halfof the paper covered with grain, the glue-sand coat tends to contractmore than the 40 paper heretofore used, with loss of moisture, so that asand concave curl is produced.

It might seem a simple matter to adjust the sand coat, that is, make acoating of sand somewhere between the closed and open coats, so as to 45match the expansion. and contraction oi the paper, and thus produce anon-curling sheet. This can be done and a sand-glue coat can be made tomatch practically any commercial paper in existence in expansioncharacteristics, but the to product produced is not suitable for thevariable sanding operations that must be performed. Di!- ierent types ofcoatings are needed for difierent abrading operations. For instance,hard wood or metal generally should be sanded with a tight 5!orclosecoatoigrainsoastoresistwearing away or the abrasive edges andgive the desired production per belt or sheet oi coated abrasives. Onthe contrary, soft, greasy leather should be sanded with an open coatingoi sand so as to leave plenty of clearance between the grains andprevent premature filling of the coated abrasive by the soft bulkymaterial that is removed, failure in such cases occurring as a result offilling between and over the grains. Thus, the many uses to which coatedabrasives are put in the art requires a wide variety of different types01' coatings, and no single type 01' paper backing is suitable i'or all.

It is a primary object 01' the present invention to produce a completeline of coated abrasives having greatly improved non-curlingcharacteristics. This object is accomplished by my new method ofcontrolling the expansion-contraction characteristics of the paperbacking sheet under moisture content changes, and relating thosecharacteristics to the corresponding characteristic of the particulartype oi adhesive abrasive coating desired.

In the case of coated abrasive belts. strong rope papers made on acylinder paper machine are normally used to meet the requirements forhigh longitudinal tensile strength and good tearing strength and toprevent the destruction of the belt on the machine. been used to someextent and it is common practitle to employ some jute in the furnish.Various mixtures of rope, wood and jute are also used.

In the manufacture of paper especially on a' cylinder machine asheretofore used, the fibres are oriented with the long direction of thefibre predominantly parallel to the running direction of the cylindermachine. The result is a relatively high length strength of the paper ascompared with the cross strength of the papers.

I have discovered that, when an individual fibre expands or contractswith gain or loss or moisture, the resulting change in its dimensions isvery largely a change in the diameter or short direction of the fibre,the length direction bein changed to a much smaller degree. I havedetermined that it is the sum total or expansions and contractions ofthe individual fibres that largely governs the expansion and contractioncharacteristics of the paper. Thus, the proportion of transverselydisposed fibres to longitudinally disposed fibres in the sheet not onlycontrols the cross to length tensile strength ratio, but also, the ratiobetween transverse and iongitudinal contraction and expansioncharacteristics of the sheet under changes in moisture content, such asmay result from changes in the humidity of the surrounding atmosphere.As the proportion of cross fibres and relative transverse tensilestrength is increased, the relative transverse contraction and expansioncharacteristic of the sheet is decreased.

If we consider, i'or example, lb. rope cylinder paper, length strengthusually varies from 130 to lbs. per inch of width of paper, whereas thecross strength, per inch of width of paper, will vary from about 50 to75 lbs. Fourdrinier papers have a much lower length strength and asomewhat increased relative cross strength. Such Fourclrinier papers arenot, however, generally so suitable for belt work because of their lowlength tensile strengths.

If we consider any specific paper, either rope or Fourdrinier, theorientation of the fibres is roughly indicated by the length and crossten- Long wood fibres have sile strengths. Cylinder papers as made inthe past for coated abrasive work have had a cross to length tensilestrength ratio falling between .31 and .52. Generally speaking, closedcoats of sandpapers with a ratio of about .45 to .52 have only amoderate degree of curl. With open coatings, the curl may be quite bad,sand concave. With closed sand coats, papers with a cross to lengthratio of .31 to .40 curl quite badly, sand convex. With open coatings,the tendency to curl sand concave is only moderate for papers showing across to length ratio of .31 to .37.

In addition to the cross to length relation, the expansion-contractioncharacteristics are controlled, somewhat, by the density of the paper. Aless dense paper will expand and contract and curl less than a moredense paper, provided the paper is not so porous that substantialpenetration oi the adhesive into the paper occurs. This means that for agiven basis weight of paper curling, will be decreased by using arelatively thick (high caliper) paper instead or a thin (low caliper)paper. Thus, 130 lb. rope cylinder paper with a caliper of 11.5 milswill cup less than 130 lb. cylinder paper with a caliper oi! 9.0 mils.Generally speaking, the thick papers have a relatively rough surface andare not, therefore,

so suitable for fine grits of coated abrasives. For this reason, it isadvisable to still further increasethe cross to length tensile ratio andobtain a flat product with the smoother and more dense paper.

Heretofore, the importance of control of fibre orientation has not beenappreciated in the coated abrasive art. Moreover, even accidentallystrong cylinder paper has not existed which met the requirements forbelts and other severe machine operations, with a cross to lengthstrength ratio in excess of .52; also, there has not been used in thepaper backed abrasive industry, a coated abrasive paper backing with across to length ratio below .31. As a result of my discoveries and bythe use of suitable equipment, such as a Bonbert Cylinder Machine, ofthe type disclosed in United States Patent 1,924,154, it is now possibleto control the cross to length strength ratios within wide limits andproduce backings tor coated abrasives which have the desired cross tolength tensile ratios and, as a result, the desired transverse andlongitudinal expansion and contraction characteristics under changes inmoisture content.

Desirable cross to length strength ratios for closed coats of sandpaperintended for use as belts or machine covers, have been found to bepreferably from .52 to .65, although in some cases there is an advantagein using a paper with a cross to length ratio in excess or .65. Forvery-open sand coatings for the same uses, a desirable ratio range ofcross to length strength is .25 to .31.

It should be understood that the curl found objectionable for belts,drum covers and similar mechanical operations is the curl about an axisparallel with the running direction of the belt or cover, as the casemay be. This is true since the belts or covers are handled in roll form,the roll being wound around an axis perpendicular to the long directionof the belt or cover. When belts or covers are made and placed upon themachine, cupping or curling about an axis perpendicular to the runningdirection is prevented by the tension that is applied to the belt orcover.

Thus, it is of maximum importance to control the cross fibre content ofthe paper and the re- 15 suiting cross tensile strength, and transverseexpansion and contraction characteristic of the sheet for belt use, sothat the latter characteristic corresponds substantially with theexpansion and contraction characteristic of the particular glue-sandcoat to be used in the finished belt. As stated above, the longitudinalfibre content, the resulting length tensile strength, and longitudinalexpansion-contraction characteris tic are not of particular importancein belt use, because of the tension under which such devices operate.The requirement for longitudinal tensile strength is satisfied, if thepaper is strong enough to withstand the tension under which it is used.Control of the cross fibre content and transverse expansion andcontraction, to correspond to expansion and contraction of the glue-sandcoat will prevent curling of the belt about an axis parallel to thelength of the belt, which is the important consideration.

When we consider coated abrasives in sheet form, sometimes known as reampaper, the material is not necessarily kept under tension in use. As amatter of fact many of these sheets are used primarily for handoperations and no tension is applied. Likewise, the sheet may be used inany direction and there is no requirement for excess strength in onedirection to the exclusion of strength in the other direction. For suchuses as this, a superior product is produced by having equal strengthsin all directions.

If we make an open coat sheet out of 8. cylinder paper with a low crossto length ratio, the sheet curls badly, sand concave, about an axisperpendicular to the machine direction of the original paper backing.Since the requirements for tensile strength are not so severe, so-calledFourdrinier papers are used in many cases for goods that are to be cutinto sheet form. In the case of Fourdrinier paper, the balance betweencross to length strength is better, ratios as high as .67 occurring inthe case of Fourdrinier papers.

Such paper has not, however, eliminated the.

curling of sheet form coated abrasives. A common form of sheet is 9" x11''. As will be apparent, it is impossible to match the expansion tionof this paper with a definite sand-glue coat.

As previously explained, no single sand weight coating will meet all therequirements of the trade. I have found that from the commercialstandpoint, the most satisfactory solution for the sheet problem is toproduce an appropriate backing with, roughly, cross to length ratio of1, and then use the sand coat best suited for the operation in question.It is true that the product will often tend to curl on one axis or theother, or to form a segment of a hemispherical surface. The curl,however, on the average, is less than is the case if the paper hasunbalanced structure which in one direction gives a greater expansion orcontraction and in the other a lower expansion or contraction withchanges in humidity, than is the case in either direction if the papermcking has a balanced structure. This can be easily understood. Many ofthese sheets approach squares or in any case, have the long and shortdimensions of the same order of magnitude. A curl on either axis may,therefore, occur with substantially equal facility, and the nearestapproach to a flat sheet is one with a balanced structure of the backinghaving an equal expansion and contraction in the two directions withchanges in humidity, since the alternate is a backing with one directionhaving a higher and the other direction a lower rate of change withchanges in humidity than is the case in either direction with thebalanced structure which has in both directions, equal but intermediaterates of change with humidity.

A balanced structure of backing for hand sheets has the furtheradvantage of a uniform strength in all directions. With hand sheets thedirection of rubbing in use is largely a matter of chance, hence ease ofbreakage is more often dependent upon the weakest direction of thepaper. With a balanced structure of paper, the strength in any directionis less than that of the stronger direction of unbalanced paper, but isappreciably greater than that of the weaker direction.

It is a common practice in the coated abrasive industry to form abacking by combining cloth and paper with an adhesive such as animalglue. I have found that the expansion characteristics of the paper withchanges in humidity very largely determine the curl of coated abrasivesmade with this combination backing, in which the abrasive and adhesivecoating is applied to either the paper or the cloth side of thecombination. It has been found that a combination backing with improvedcurling characteristics is produced by selecting paper in accordancewith the principles heretofore laid down for selecting paper backings.Where I refer to "a paper backing" in the specification and claims, Iintend to cover a cloth paper combination backing, as well as a backingconsisting solely of paper.

It is also common practice to use as a backing for coated abrasives,especially for discs that are to be used in leveling welds andprojecting edges in metal work, a backing consisting either ofvulcanized fibre or of vulcanized fibre combined with cloth, with anadhesive such as animal glue. By vulcanized fibre" which I intend toinclude by the broad term "paper", is meant, a paper of commerciallypure cellulose usually made of rags or oihighly purified cellulosesuchas alpha fibre, which has been treated with zinc chloride, sulphuricacid, calcium sulphocyanate or similar solutions well known in the art,to produce a highly hydrated, relatively stifl, and dense product. Ihave found that discs made by coating either fibre or fibre-clothcombination, have a much smaller tendency to curl or cup if the fibrehas a high cross to length tensile strength ratio. The ideal backing isformed by using fibre with a cross to length strength tensile ratio ofsubstantially 1. Not only is such a ratio advantageous in preventingcurl but since the material is used in the form of a disc, therequirements for tensile strength are the same in all directions. It hasfurther been determined that the higher the cross to length. tensilestrength ratio of the paper used for making the vulcanized fibre, thehigher the same ratio for the finished vulcanized fibre. While it isvery difiicult to produce a vulcanized fibre with a cross to lengthratio equal to 1, it is possible to more closely approach this ratiothan has been the case in the past. Heretofore, it has not been thepractice to make vulcanized fibre with a cross to length tensile ratioin excess of .52, but by using paper with a high cross to lengthstrength ratio, it has been found possible to produce vulcanized fibrewith a ratio well in excess of .52. Where I refer to a vulcanized fibrebacking in the specification and claims, I intend to cover acloth-vulcanized fibre combination backing as well as a backingconsisting solely of vulcanized fibre.

In cases where a synthetic phenolic resin or other relativelynon-hygroscopic adhesive is used to replace the relatively hygroscopicadhesives, such as animal glues, the paper, combination, or fibrebacking expands and contracts to an appreciable extent, 1. e.,substantially the usual extent, with changes in humidity, but theadhesive changes in dimensions to only a negligible extent with changesin humidity. In such cases, it is advantageous to use as a backing, orin making combination backing, paper or fibre with the lowest rate ofchange in dimensions with humidlty (highest cross to length tensileratio), consistent with other requirements, 1. e., breaking tensilestrength in length direction for belts and the like.

From the point of view of both method and article, the present inventionhas an important bearing on coated abrasives which are made inaccordance with the recently developed electrostatic and magneticmethods of depositing and orienting the abrasive grains upon anadhesively coated backing sheet. In these methods great pains are takenaccurately and carefully to orient the grains on the sheet, inpredetermined relation to each other, and with their major axesperpendicular to the sheet. Obviously, some of the advantages whichmight be expected to resuit from such orientation or the grains arelost, if their position with respect to the sheet and the work issubsequently changed. When an abrasive sheet curls in accordance withchanges in atmospheric conditions, the relation of the grains to oneanother and to the backing sheet is necessarily altered. Such curling ofthe sheet and the changing of the angles of the grains is undesirable inspecially oriented sandpaper articles.

Thus, the present invention, which results in a non-curling sandpaper,improves the product made by the recently developed electro-static andmagnetic depositing and orienting means, because it results inmaintaining the orientation, and the predetermined positioning of theabrasive grains accomplished by such methods.

Where cross to length tensile strength ratio is used in thesespecifications, it should be understood that this ratio is obtained bydividing the cross tensile strength by the length tensile strength. Thelength direction is what is known as the machine direction. The crosstensile strength is obtained by testing 1" strips of paper cutperpendicular to the machine direction. The length tensile is obtainedby testing I" strips which are cut parallel to the machine direction.All tests are made at a relative humidity of 65% and a temperature of 70F., after the paper has been conditioned at this humidity andtemperature for at least two hours. A paper testing machine, motordriven, such as a Scott machine, is used. At least three samples areused for each determination, The testing machine is motor driven. Thedistance between jaws of the testing machine is 5", and the speed oftravel of the testing machine .is from 7 to 8 inches per minute. Samplesthat break in the jaws of the testing machine should be rejected.Results used as cross and length tensile strengths respectively, are theaverage of three tests in each case.

It should also be understood that where the term cylinder paper" isused, the paper may be formed on one cylinder to produce a one plycylinder paper, or on two or more cylinders to produce a multiple plycylinder paper.

I claim:--

1. A coated abrasive comprising a paper backing and an adhesive abrasivecoating thereon, said paper backing having its fibres so selectivelyoriented that the selected resultant ratio of its length strength to itscross strength is such as to make the expansion and contractiondifferential between the paper and the ahesive abrasive coating in thepresence of humidity changes so predeterminedly small that the coatedabrasive is rendered substantially completely non-curling when exposedto variable humidities.

2. An open coated abrasive article comprising a paper backing and anadhesive abrasive coating theeron, said paper backing having its fibresso oriented that the ratio of its length strength to its cross strengthis such as to make the expansion and contraction difierential betweenthe paper and the adhesive abrasive coating in the presence of humiditychanges so small that the coated abrasive is rendered substantiallycompletely non-curling when exposed to variable humidities.

3. A close coated abrasive article comprising a paper backing and anadhesive abrasive coating thereon, said paper backing having its fibresso oriented that the ratio of its length strength to its cross strengthis such as to make the expansion and contraction difierential betweenthe paper and the adhesive abrasive coating in the presence of humiditychanges so small that the coated abrasive is rendered substantiallycompletely non-curling when exposed to variable humidities.

4. An abrasive belt comprising a paper backing and an adhesive abrasivecoating thereon, said paper backing having its fibres so oriented thatthe ratio of its length strength to its cross strength is such as tomake the expansion and contraction differential transversely of the beltbetween the paper and the adhesive abrasive coating in the presence ofhumidity changes so small that the coated abrasive is substantiallycompletely non-curling transversely of the belt when exposed to variablehumidities.

5. A coated abrasive comprising a paper backing and an adhesive abrasivecoating thereon, said paper backing having its fibres oriented toproduce substantial equality in cross to length strength ratio and incross to length ratio of expansion-contraction characteristics underhumidity changes, said composite adhesive abrasive coat having anexpansion-contraction characteristic under humidity changes which isequal in all directions so that the expansion and contractiondiilerential between the paper and the adhesive abrasive coating in thepresence of humidity changes is so small that the coated abrasive isrenderedrelatively non-curling when exposed to variable humidities.

6. A coated abrasive article such as an elongated sandpaper beitcomprising a backing sheet of cylinder paper and a close coat ofabrasive grains adhesively united thereto, said paper backing having itsfibres so oriented that the ratio of its length strength to its crossstrength is within the range of substantially 0.52 to 0.05 and is suchas to make the expansion and contraction differential transversely ofthe belt between the paper and theadhesive abrasive coating in thepresence of humidity changes so small that the coated abrasive isrelatively non-curling transversely of the belt when exposed to variablehumidities.

'l. A coated abrasive article such as an elongated sandpaper beltcomprising a backing sheet of paper and an open coat of abrasive grainsadhesively united thereto, said paper backing having its fibres sooriented that the ratio of its length strength to its cross strength iswithin the range of substantially 0.25 to 0.31 and is such as to makethe expansion and contraction differential transversely of the beltbetween the paper and the adhesive abrasive coating in the presence ofhumidity changes so small that the coated abrasive is relativelynon-curling transversely of the belt when exposed to variablehumidities.

B. A coated abrasive comprising a paper backing and an adhesive abrasivecoating thereon, said paper backing having its fibres so oriented thatthe ratio of its length strength to its cross strength is within therange of substantially 0.70 and 1.0 and having transverse andlongitudinal contraction and expansion characteristics whichproportionately approach equality so as to make the expansion andcontraction diilerential between the paper and the adhesive abrasivecoating in the presence oi humidity changes so small that the coatedabrasive is rendered relatively non-curling when exposed to variablehumldities.

9. A coated abrasive having a paper backing comprising vulcanized fibreand an adhesive abrasive coating thereon, said paper backing having itsfibres so oriented that the ratio 01 its length strength to its crossstrength is in excess of 0.52 andis such as to make the and contractiondiii'erential between the paper and the adhesive abrasive coating in thepresence of humidity changes so small that the coated abrasive isrelatively free from curling when exposed to variable humiditles.

10. A coated abrasive comprising a paper backing and an adhesiveabrasive coating thereon, said adhesive being non-hygroscopic. the paperbacking having its fibres so oriented that the ratio 01' its lengthstrength to its cross strength is substantially within the range of 0.10and 1.0 and is such as to make the expansion and contractiondiflerentlal between the paper and the adhesive abrasive coating in thepresence of humidity changes so small that the coated abrasive isrelatively free from curling in all directions when exposed.to variablehumidities.

11. A coated abrasive comprising a paper backing and a synthetic resinadhesive abrasive coating thereon, said adhesive being non-hygroscopic,the paper backing having its fibres so oriented that the ratio of itslengthstrength to its cross strength is substantially within the rangeof 0.70 and 1.0 and is such as to make the expansion and contractiondiflerential between the paper and the adhesive-abrasive coating in thepresence of humidity changes so small that the coated abrasive isrelatively free from curling in all directions when exposed to variablehumidities.

12. A coated abrasive comprising a paper backing and a synthetic resinadhesive abrasive coating thereon, said paper backing having its fibresso selectively oriented that the selected resultant ratio of its lengthstrength to its cross strength is such as to make the expansion andcontraction diil'erential between the paper and the adhesiveabrasivecoating in the presence of humidity changes so predeterminedly smallthat the coated abrasive is rendered substantially completely noncurlingwhen exposed to variable humiditles.

13. An open coated abrasive article comprising a paper baclnng and anadhesive-abrasive coating thereon, said paper backing having its nbresso oriented that the ratio of its length strength to its cross strengthis within the range of substantially 0.25 to 0.31 and is such as to makethe expansion and contraction differential in the cross directionbetween the paperand the adhesive-abrasive coating in the presence oihumidity changes so small that the coated abrasive is renderedsubstantially completely non-curling on an axis parallelto the machinedirection when exposed to variable humidities.

14. Aclosecoatedabrasivearticle i apaperbackingofcylinderpaperandanadhesiveabrasive coating thereon, saidpaper backing havingitsflbressoorientedthattheratiootitslengthstrengthtoits crossstrengthiswlthinthe rangeofsubstantiallymfiiito0.65andissuchas to make the expansion and contractiondflierentialinthecrossdirectionbetweenthepaperand adhesive-abrasivecoating in the presence 01' humiditychangessoamallthatthecoatedabrasiveis rendered substantially completely non-curling on an axis parallel tothe machine direction when exposed to variable humiditles.

NICHOLAS EWING OGLEBY.

